SyntaxError: a declaration in the head of a for-of loop can't have an initializer
The JavaScript exception "a declaration in the head of a for-of loop can't have an initializer" occurs when the head of a for...of loop contains an initializer expression such as for (const i = 0 of iterable)
. This is not allowed in for-of loops.
Message
SyntaxError: for-of loop variable declaration may not have an initializer. (V8-based) SyntaxError: a declaration in the head of a for-of loop can't have an initializer (Firefox) SyntaxError: Cannot assign to the loop variable inside a for-of loop header. (Safari)
Error type
What went wrong?
Examples
Invalid for-of loop
js
const iterable = [10, 20, 30];
for (const value = 50 of iterable) {
console.log(value);
}
// SyntaxError: a declaration in the head of a for-of loop can't
// have an initializer
Valid for-of loop
You need to remove the initializer (value = 50
) in the head of the for-of
loop. Maybe you intended to make 50 an offset value, in that case you could add it to the loop body, for example.
js
const iterable = [10, 20, 30];
for (let value of iterable) {
value += 50;
console.log(value);
}
// 60
// 70
// 80
See also
for...of
for...in
– disallows an initializer in strict mode as well (SyntaxError: for-in loop head declarations may not have initializers)for
– allows to define an initializer when iterating.